Electrocardiography, or ECG, involving the collection and study of the electrical activity from the heart, has long been an effective method for the diagnosing and treatment of problems or irregularities related to the operation of the heart. Generally, ECG is used for two major purposes: (1) for diagnosing cardiac arrhythmia's; and (2) providing information on the status of the myocardium.
The early methods for obtaining an electrocardiogram or ECG included the placing of electrodes invasively through the skin or noninvasively attaching the electrodes to the surface of the patient's skin, such as the chest or limbs, by adhesion or clamping. However, certain anatomical disabilities of the human or animal physiology often obstruct the hearts electrical current from the measurement device and thus cause an inaccurate measurement. To avoid these problems, methods were developed for conducting invasive electrocardiograms via the esophagus region.
Esophageal probes for monitoring a patient are, in general known. For example, U.S. Pat. No. RE 31,377, Mylrea et al., reissued Sep. 13, 1983, and U.S. Pat. Nos. 4,349,031 and 4,476,872, Perlin, issued Sep. 14, 1982 and Oct. 16, 1984, respectively, disclose catheters used for monitoring the patients electrocardiogram, heartbeat sounds and temperature. However, disadvantages exist with these probes due to the use of either pill or ring electrodes. Pill electrodes, as shown in FIG. 1A, which are electrodes capable of being swallowed by the patient in an attempt to position the electrodes in the esophagus at the level of the atria, have yielded results that were inconclusive because of variability in electrode placement. Ring electrodes, generally comprised of a conductive band wrapped around the circumference of the probes' flexible tubing, as shown in FIGS. 1B and 1C, can float within the esophagus, and thus, have also yielded inconclusive results for similar reasoning. In particular, when the electrode is not in contact with the tissue wall of the esophagus directly, a fluid or mucosal connection affects the impedance of the received signals, and thus, detrimentally impacts the diagnostic quality of the signals.
Another example, U.S. Pat. No. 3,951,136, Wall, issued Apr. 20, 1976, also discloses an esophageal probe used for monitoring a patient's cardiac electrical activity, heart sounds and temperature wherein the probe disclosed utilizes a pair of spaced electrodes with domed-shaped outer heads. Although these electrodes purport to provide improved contact surface over previously described electrodes, the disclosed electrodes limit and restrict the rotational position of the probe within the esophagus. Moreover, the electrodes configuration suffers from inherent design inadequacies. Further, like the other prior art esophageal probes, the distal end of the probe comprises a thin membrane or diaphragm that seals the end of the probe. As one skilled in the art will appreciate, these thin-walled diaphragms, while effective in allowing for temperature and sound measurements to be obtained, are quite fragile and are often damaged during insertion and use within the patient's esophagus.
Other methods for evaluating cardiac functions are known in the prior art. One particular example, impedance cardiography, is increasingly an important mechanism for determining a patient's cardiac condition both during and following medical procedures.
Impedance cardiography falls within the more general category of impedance plethysmography, which refers to the measurement of volume (and thereby flow) changes in the body, as derived from observing changes in electrical impedance. Impedance cardiography, generally, is a noninvasive bioimpedance method for measuring cardiac output. Specifically, cardiac output measurements are based on the principal that blood is a conductor of electricity and that the electrical impedance of the thorax will change during a cardiac cycle. This change in impedance is caused by the thoracic aortic blood flow which is directly related to the amount of blood ejected from the heart.
U.S. Pat. No. 3,340,867, now Re. 30,101, reissued Sep. 1979 to Kubicek, et al., discloses a method for determining cardiac output by measuring the patient's heart stroke volume. There, an impedance plethysmograph employs two sets of electrodes placed on the neck and chests of patients, to provide an impedance difference signal from the two center electrodes. A constant, low-amplitude, high-frequency alternating current is applied to the outermost pair of electrodes while the innermost pair of electrodes senses the voltage levels above and below the patient's heart. Kubicek et al.'s method entails first determining the heart stroke volume from these impedance signals, based on the observation that resistance to a current passed through the chest varies with thoracic aortic blood flow, and from this determination of stroke volume, then estimating the cardiac output.
U.S. Pat. No. 4,450,527, issued to Sramek on May 22, 1984, generally discloses a similar apparatus, model and equation for relating impedance and stroke volume to determine cardiac output. U.S. Pat. No. 5,309,917, issued May 10, 1994, U.S. Pat. No. 5,423,326 issued Jun. 13, 1995, and U.S. Pat. No. 5,443,073 issued Aug. 22, 1995, all of which were issued to Wang, et al., each generally disclose variations of the Kubicek and Sramek methods.
Yet another model and method of impedance cardiography regarding the placement and spacing of electrodes has been proposed by Bernstein. According to Bernstein, stroke volume (SV) is related to the change in impedance (Z) as shown in Equation 1: ##EQU1##
SV=Stroke Volume PA1 .delta.=correction factor for patient weight PA1 H=Patient height (cm) PA1 T.sub.LVE =left ventricular ejection time (sec) PA1 (dZ/dt).sub.max =maximum value of the first derivative of Z, where Z is the PA1 change in impedance caused by thoracic aortic blood flow PA1 Z.sub.0 =mean baseline impedance of the thorax (ohm)
While each these methods can be helpful in determining cardiac output, the various types of non-invasive devices disclosed such as the outer skin electrodes of Kubicek and Sramek, often prove inefficient, for example when dealing with many surgical procedures or with skin abrasion patients. As one can imagine, these devices require a number of exposed connective wires and corresponding electrodes that may interfere with other surgical procedures. Furthermore, because the inner surface electrodes may receive impedance signals from various other regions within the patient due to the distance in placement of the electrodes from the thoracic aorta region, accuracy concerns have been raised. Additionally, incorrect electrode placement can result due to the changes in the patient's physiology of the thorax with respect to the placement of the electrodes on the sternum, as well as due to the size of the patient. Finally, as recognized in Equation 1, a correct factor for patient weight, .delta., must be utilized in calculating cardiac output, and often if the weight cannot be accurately determined, the weight estimation can be another source of inaccuracy.
Several of the problems with prior art non-invasive devices have been addressed by more recent developments; however, these new developments still fall short in many critical areas. For example, U.S. Pat. No. 4,836,214, issued to Sramek on Jun. 6, 1989, generally relates to an esophageal probe comprised of an array of electrical bioimpedance ring electrodes provided on a hollow, flexible tube that is insertable into the esophagus of a patient and positioned proximate the descending thoracic aorta. The Sramek device, however, like other non-invasive prior art probes, still permits movement of the probe within the esophagus. As a result of this motion, artifact inaccuracies are possible. This problem is further attenuated by the use of the ring electrodes in that such electrodes often tend to float within the esophagus, as stated previously above.
U.S. Pat. No. 5,357,954, issued to Shigezawa et al. on Oct. 25, 1994, generally relates to an esophageal blood oxygen saturation probe with temperature and sound sensing devices for invasively monitoring a patient. The patent purports to suggest discloses that the internal walls of the esophagus will tend to collapse onto the outer surface of the probe's chassis and sound sensor, such that the probe's sensors will not move appreciably with respect to the esophagus. The ability of the esophagus to prevent undesirable movement of the probe as so disclosed, particularly given the size of the probe, is questioned. Nevertheless, because the probe is not substantially fixed relative to the esophagus, there still exits an opportunity for undesirable movement which, as will be appreciated by those skilled in the art, can lead to inefficient and less accurate results.
Motion limiting devices such as those disclosed in prior oximetry work of the present assignee are known; however, heretofore teachings have not been used in cardiac evaluations, such as impedance cardiography applications. In this regard, the subject matter of application Ser. No. 60/045,006, application Ser. No. , application Ser. No. 08/1546,246 (Pat. No. 5,715,816), application Ser. No. 08/412,287 (Pat. No. ) and Pat. No. 5,417,207 are incorporated herein by reference.
Thus, there exists a long felt need, for an electrode configuration, such as for use in connection with an esophageal probe, which addresses the various deficiencies of the configurations shown in the prior art as discussed herein, including, among other things, inaccurate readings, difficulty in manufacture, reliability in use, fragile construction, and the like. Moreover, there exists a long felt need for an esophageal probe to provide electrodes that remove the variability of electrode placement and contact and provide a concise method of measurement from the esophagus region. Additionally, there exists a need for an esophageal probe that can obtain sound and temperature measurements while evaluating various cardiac functions without the potential for becoming damaged during insertion and use within the patient's esophagus.